Section 35 ILCS 5/1401 - Promulgation of Rules and Regulations(a) In general. The Department is authorized to make, promulgate and enforce such reasonable rules and regulations, and to prescribe such forms, relating to the administration and enforcement of the provisions of this Act, as it may deem appropriate.(b) Group administration for taxpayers that are members of a unitary business group. (1) For taxable years ending before December 31, 1993, the Department shall make, promulgate and enforce such reasonable rules and regulations, and prescribe such forms as it may deem appropriate, to permit all of the taxpayers that are corporations (other than Subchapter S corporations) having the same taxable year and that are members of the same unitary business group to elect to be treated as one taxpayer for purposes of any original return, amended return which includes the same taxpayers of the unitary group which joined in the election to file the original return, extension, claim for refund, assessment, collection and payment and determination of the group's tax liability under this Act. For taxable years ending on or after December 31, 1987, corporate members (other than Subchapter S corporations) of the same unitary business group making an election to be treated as one taxpayer are not required to have the same taxable year. The rules, regulations and forms promulgated under this subsection (b) shall not permit the election to be made for some, but not all, of the purposes enumerated above.(2) For taxable years ending on or after December 31, 1993, the Department shall make, promulgate and enforce such reasonable rules and regulations, and prescribe such forms as it may deem appropriate, to require all taxpayers that are corporations (other than Subchapter S corporations) and that are members of the same unitary business groups to be treated as one taxpayer for purposes of any original return, amended return, which includes the same taxpayers of the unitary group which joined in filing the original return, extension, claim for refund, assessment, collection and payment and determination of the group's tax liability under this Act.(c) Offset among taxpayers that are members of a unitary business group. For taxable years for which returns were filed prior to the applicable date of Section 502(f), the Department shall make, promulgate and enforce such reasonable rules and regulations, and prescribe such forms as it may deem appropriate, to permit a taxpayer that is a member of a unitary business group to elect, within the applicable period of limitation as provided in Section 911, to credit any overpayment due the taxpayer for a taxable year against the liability for the same taxable year of one or more other taxpayers that are members of the same unitary business group for that taxable year, except that when an audit has been conducted by the Department, overpayments determined by the Department to be due a taxpayer may be credited against the liability of one or more other members of the same unitary group for any year within the period covered by the audit. Such regulations shall include rules which provide that the amount of the overpayment taken as a credit by a taxpayer under this Section shall be treated, for all purposes under this Act, as having been paid by such taxpayer at the time such payment was made.